Tapered optical fibers and fused couplers are widely used as low loss all-fiber components in optical fiber communications systems. The typical method of fabricating tapered and fused fiber devices involves heating the fibers in a small flame to soften them as they are drawn and fused. The characteristics of the flame used to soften the optical fibers effects the uniformity of the taper, and therefore the performance of the device. Flames produced by burners have height and width dimensions, which vary in size according to the diameter of the burner nozzle, flow rate of gasses and gas mixture, and, the temperature at various positions within flames of different size is not the same. In order to ensure fabrication repeatability and device uniformity, tapered optical fibers and fused couplers should be manufactured using the most thermally stable part of the flame. It is therefore important to accurately determine the spatial temperature distribution of the flame so that the burner position setting(s) can be calibrated for the fabrication process.
Various techniques have been employed in the past which could be used to either measure the flame temperature directly, or indirectly by measuring the heat radiated from an optical fiber subjected to the flame. For example, spectropyrometers such as disclosed in U.S. Pat. Nos. 6,379,038 and 5,772,323 collect the emitted blackbody radiation from a heated body and use spectral processing to determine the emissivity as a function of wavelength and the absolute temperature of the blackbody. Glass, used to form optical fibers, has low emissivity and radiated light from heated optical fibers is difficult to detect using a spectropyrometer.
Another device for measuring temperature is a thermocouple, such as disclosed in U.S. Pat. Nos. 6,857,776 and 6,632,018. Thermocouples operate based upon the thermoelectric effect at the junction of two dissimilar metals. In response to the application of heat, a voltage is generated across the junction which is proportional to the temperature. The emissivity and conductivity of the metals used in thermocouples is very different from that of glass fibers. Additionally, at the temperature range of interest, e.g., 1700° C. to 1900° C., temperature measurements of thermocouples have a large degree of uncertainty.
Single and multiple wavelength pyrometers are also employed to measure temperature. Devices of this type use infrared radiation to measure temperature, but the accuracy of such measurements is dependent on knowledge of the emissivity of the target material. Emissivity is a property which changes during the heating cycle of most materials, and, as noted above, glass has low emissivity. These factors make it difficult for pyrometers to provide an accurate measurement of the flame temperature applied to an optical fiber.